Radiolabeled anti-CD20 antibodies produce responses in 60% to 95% of patients with relapsed non-Hodgkin lymphoma (NHL); however, absorbed radiation ratios between tumors and normal organs are relatively low, and many patients have relapses. In this study we compared the abilities of anti-CD45 (BC8) and anti-CD20 (1F5) antibodies to target human Ramos lymphoma xenografts in athymic mice. When direct radioiodination was performed with conventional methods, BC8 delivered 2-to 4-fold more radioiodine to tumors than 1F5, with tumor-to-normal organ ratios as high as 20:1 using radiolabeled BC8 compared with a maximal ratio of 9.8:1 using radioiodinated 1F5. To optimize the biodistribution of radioactivity, we performed studies following a pretargeting method using streptavidin (SA)-conjugated BC8 and 1F5. Injection of a synthetic clearing agent decreased the circulating level of conjugates by 80% to 90% within 1 hour. Pretargeting with BC8-SA resulted in a 2-to 4-fold greater tumor uptake of radiolabeled biotin than with 1F5-SA, with maximal tumor-to-normal organ ratios of more than 80:1 and approximately 16:1, respectively. Therapy experiments demonstrated that 400 Ci (14.8 MBq) or more of yttrium-90-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)-biotin cured 100% of mice treated with BC8-SA and more than 90% of mice pretargeted with 1F5-SA, with complete remission occurring 8 to 10 days sooner in mice receiving BC8-SA. After treatment with 200 Ci (7.4 MBq) 90 Y-DOTA-biotin, 70% of the mice treated with BC8-SA were cured, but no mice were cured using 1F5-SA. Doses up to 800 Ci (29.6 MBq) 90 Y-DOTA-biotin were delivered with minor toxicity using either antibody conjugate. These lymphoma xenograft data suggest that pretargeted radioimmunotherapy using either anti-CD20 or anti-CD45 conjugates is highly effective and minimally toxic. (Blood.

Acute myelogenous leukemia (AML) currently kills the majority of afflicted patients despite combination chemotherapy and hematopoietic cell transplantation (HCT). Our group has documented the promise of radiolabeled anti-CD45 monoclonal antibodies (Ab) administered in the setting of allogeneic HCT for AML, but toxicity remains high, and cure rates are only 25% to 30% for relapsed AML. We now show the superiority of pretargeted radioimmunotherapy (PRIT) compared with conventional radioimmunotherapy using a recombinant tetravalent single-chain Ab-streptavidin (SA) fusion protein (scFv 4 SA) directed against human CD45, administered sequentially with a dendrimeric N-acetylgalactosamine-containing clearing agent and radiolabeled 1,4,7,10-tetraazacyclododecane-N,N V ,N 00 ,N V 00 -tetraacetic (DOTA)-biotin. The scFv 4 SA construct was genetically engineered by fusing Fv fragments of the human CD45-specific BC8 Ab to a full-length genomic SA gene and was expressed as a soluble tetramer in the periplasmic space of Escherichia coli. The fusion protein was purified to >95% homogeneity at an overall yield of f50% using iminobiotin affinity chromatography. The immunoreactivity and avidity of the fusion protein were comparable with those of the intact BC8 Ab, and the scFv 4 SA construct bound an average of 3.9 biotin molecules out of four theoretically possible. Mouse lymphoma xenograft experiments showed minimal toxicity, excellent tumor-specific targeting of the fusion protein and radiolabeled DOTA-biotin in vivo, marked inhibition of tumor growth, and cured 100% of mice bearing CD45-expressing tumors. These promising results have prompted large-scale cGMP production of the BC8 fusion protein for clinical trials to be conducted in patients with hematologic malignancies. (Cancer Res 2006; 66(7): 3884-92)